The present invention relates to methods of preparing polyester molding compositions. More specifically, the present invention relates to methods of preparing reinforced polyester molding compositions, and still more particularly to methods of preparing improved "low-profile" molding compositions.
Reinforced molding compositions containing unsaturated polyesters and high proportions of reinforcing agents (such as glass fibers) and fillers are well known in the art. It has also been known to modify these compositions by employing a thermoplastic additive, such as poly (methyl methacrylate), polyvinylacetate, or polystyrene, in order to provide a two-phase system which has improved surface appearance and other physical properties. These compositions have been employed as both sheet molding and bulk molding compounds for the fabrication of various products therefrom, and are known to possess high strength and rigidity when used for these purposes. For example, U.S. Pat. Nos. 3,701,748 and 3,722,241 both assigned to Rohm and Haas Company, disclose liquid polymerizable compositions comprising unsaturated polyesters, thermoplastic polymers, and free radical polymerizable monomers. These materials have, however, been of limited value in connection with many large volume applications because of their deficiencies in impact resistance, flexibility and resilience. In general, these thermoplastic additive modified sheet or bulk molding compounds have been referred to as "low profile" systems because the finished articles are characterized by having reduced surface roughness, which is an otherwise common result of shrinkage in the molding of such reinforced polyester compositions. The thermoplastic additives have thus been referred to as anti-shrink additives.
Attempts have been made to improve the impact resistance and flexibility of these compounds by the addition of elastomeric additives thereto. However, because these materials are grossly incompatible both with the unsaturated polyester resin itself as well as the anti-shrink additive, this has not been possible. For example, it has not been possible to retain the polyester-elastomer mixture dispersed in monomer for a sufficient time to prepare and fabricate the composition, and the inhomogeniety of the blend has been reflected in the resultant poor physical properties and lack of uniform appearance in the cured products.
As an example of the above, U.S. Pat. No. 4,020,036, assigned to Phillips Petroleum Company, describes the difficulties in attempting to provide unsaturated polyester compositions containing rubber, and then goes on to disclose a thermosetting polymer composition comprising an unsaturated polyester, a vinyl monomer, a catalyst, a reinforcing agent or filler, and a normally solid carboxy-containing block co-polymer, such as a carboxylated block copolymer of butadiene and styrene.
A relatively recent development in connection with elastomers concerns conjugated diene butyl elastomeric copolymers, such as those described in U.S. Pat. No. 3,816,371, which issued on June 11, 1974 to Baldwin et al, and U.S. Pat. No. 3,775,387, which issued on Nov. 27, 1973 to Baldwin. Furthermore, Application Ser. No. 619,294 filed on Oct. 3, 1975 by Irwin Gardner, teaches a method of curing such elastomers with monomer absorbed onto a granular absorbant and dispersed into the elastomer.
It has not heretofor been possible, however, to produce the referred-to unsaturated polyester molding compounds including such elastomers therein, primarily due to the incompatibility of these materials as discussed above.